Radioimmunoassay of serum cortisol is simplified by a unique solution to the problem of eliminating competition between cortisol-binding globulin and cortisol immunoglobulin. Increasing incubation temperature tends to release cortisol from its carrier protein while favoring the immunoglobulin reaction. We applied this methodology, using reagents supplied as a commercial kit. Serum, 5 mul, is diluted with 125I-containing(hydroxymethyl)aminomethane buffer and incubated at 45 degrees C for 1 h, then aspirated. All standards are constituted in steroid-deprived serum. The clinical validity of the method is supported by comparisons with conventional methods during months of use under routine laboratory conditions. The simplified method also offers improved performance characteristics and, as described, if fully instrumented. The significance of this use of thermodynamic properties is discussed in terms of more general applicability.